Rapid progress has been made in the development of fiber optical communication systems, and, as a result, such systems are now commercially available, both for short-haul as well as for long-haul applications.
A particular and promising application of optical fiber is in the role of high capacity data bus in multiple-access communications systems, for instance, in on-premises installations, also called local area networks, in which many computer terminals or other input/output devices are linked to a central processing unit or high capacity data link, and to each other. For this and other applications of optical fiber, it is often necessary to access the signal carried by a fiber at intermediate points along the fiber, and/or to inject signal radiation into the fiber at intermediate points. Substantial efforts have been expended in the past on developing means (to be referred to as "taps") for this purpose. Before discussing the results of these efforts, however, I will briefly review some relevant aspects of the transmission of electromagnetic radiation by optical fibers. A comprehensive review of the optical fiber communications field can, for instance, be found in Optical Fiber Telecommunications, edited by S. E. Miller and A. G. Chynoweth, Academic Press, 1979.
Optical fibers guide electromagnetic radiation, typically radiation in the visible or infrared part of the spectrum, by utilizing the phenomenon of total internal reflection. As is well known, electromagnetic radiation, traveling in a medium having an index of refraction n.sub.1, is reflected totally at an interface with a medium having a refractive index n.sub.2 if n.sub.1 &gt;n.sub.2 and the angle of incidence at the interface is below a critical angle, the magnitude of which depends on n.sub.1 and n.sub.2.
Optical fiber typically is substantially longitudinally uniform, with a relatively high-index, central region, often referred to as the core, surrounded by a relatively low-index region, the cladding. The radial refractive index profile of optical fiber can show an abrupt change at the corecladding interface, or it can have more complicated shape. Appropriately designed fibers can guide one or more modes of radiation of the appropriate wavelength, yielding single mode or multimode fiberguides, respectively.
Although the greatest part of the electromagnetic radiation guided by an optical fiber is confined to the core, a small part of the total radiation field, the so-called evanescent wave, exists in the cladding region adjacent to the core. The signal being transmitted through the fiber in the form of modulated electromagnetic radiation (typically pulses) thus can, in principle, be accessed outside of the core region, and many prior art methods for tapping optical fiber make direct use of this fact. For instance, U.S. Pat. No. 4,054,366, issued Oct. 18, 1977, to M. K. Barnoski et al, discloses a fiber optic coupler in which a second fiber is fused longitudinally to a first fiber, thereby permitting coupling of evanescent waves from one fiber into the other. And U.S. Pat. No. 3,982,123, issued Sept. 21, 1976 to J. E. Goell et al, discloses an optical fiber tap comprising a dielectric body brought into contact with a section of the fiber from which all or most of the cladding material has been removed, thereby facilitating coupling of evanescent waves into the coupling body.
A somewhat different approach is taken by those prior art methods which locally alter the guiding properties of the optical fiber by external means, such that a part of the energy in the guided core modes is locally transferred into the cladding, and/or is radiated, or otherwise removed therefrom. For instance, U.S. Pat. No. 3,931,518, issued Jan. 6, 1976 to S. E. Miller, teaches a optical fiber tap comprising means for causing a transition of a portion of the signal power in the fiber from lower order core modes to higher order modes. The mode coupling means are exemplified by a pair of corrugated plates pressed against the fiber to periodically deform a region of the fiber just preceding the fiber tap. And U.S. Pat. No. 4,253,727, issued Mar. 3, 1981 to L. Jeunhomme et al, discloses a fiber tap comprising a grating to force the fiber into undulating shape, causing light to be radiated from the undulating portion of the fiber.
A related approach is disclosed in U.S. Pat. No. 4,270,839, issued June 2, 1981 to M. A. Cross, showing means for inducing bends in an optical fiber, resulting in radiation of optical signal power from the fiber. And U.S. Pat. No. 4,146,298, issued Mar. 27, 1979, to P. S. Szczepanek, discloses an optical fiber tap in which energy is caused to leak from the core into the cladding by raising the refractive index of the cladding in the tap region, such as by ion implantation, and detecting radiation from the fiber in the tap region.
U.S. Pat. No. 4,307,932, issued Dec. 29, 1981 to G. Winzer, discloses, inter alia, somewhat different means for extracting a signal from an optical fiber. In particular, FIG. 1B of that patent discloses a notch in the fiber cladding, intended to cause scattering of radiation from the guiding region of the fiber into the space surrounding the fiber, where the scattered radiation can be detected and transformed into an electrical signal in the usual way.
As can be seen from the above discussion, prior art optical fiber taps typically require physical, e.g., mechanical, modification of the fiberguide subsequent to fabrication of the fiber. This modification generally is carried out in situ, i.e., after installation of the fiberguide and determination of the tap site. Typically, this entails deforming the fiber, or otherwise modifying the fiber, e.g., by notching or ion implantation, or by fusing of another fiber thereto. The prior art approaches often result in weakening of the fiberguide, and, furthermore, may require some delicate and/or complex treatment or procedure (e.g., notching of the cladding with a laser) that may not easily be carried out in the field. Prior art approaches also typically do not lend themselves readily to post-installment change of tap location, and typically are not "reciprocal", i.e., do not permit injection as well as removal of radiation. A method not subject to these shortcomings, i.e., a method for tapping signal radiation from an optical fiberguide that does not require in situ mechanical modification of the fiber, with embodiments that allow substantially arbitrary placement of tap sites, that does not appreciably reduce the mechanical strength of the fiber, and that can, in appropriate embodiments, yield reciprocal taps, would, inter alia, greatly enhance the versatility and ease of installation of multiple-accessed optical fiber communications systems, and is, therefore, of considerable technological and commercial interest. Such a method is the subject of the instant invention.